This invention relates generally to wireless communication systems and in particular to a method of random access control channel synchronization compatible with time-slot synchronization and signal gain control for in-band translators deployed in peripheral cells of TDMA systems to extend the range of broadband Base Transceiver Systems (BTSs) in a cellular communication system.
The demand for wireless communication services, such as Cellular Mobile Telephone (CMT), Personal Communication Services (PCS) and the like, typically requires the operators of such systems to serve an increasing number of users. As a result, a type base station equipment known as a MULTICARRIER broadband Base Transceiver Systems (BTS) has been developed which is intended to service a relatively large number of active mobile stations in each cell. Such broadband BTS equipment can typically service ninety-six simultaneously active mobile stations, at a cost of less than $2000 to $4000 per channel in 1998 dollars.
When coupled with efficient frequency reuse schemes, such as that described in U.S. Pat. No. 5,649,292 entitled xe2x80x9cA Method For Obtaining Times One Frequency Reuse in Communication Systemsxe2x80x9d issued to John R. Doner and assigned to Air Net Communications Corporation, who is the assignee of the present application, maximum efficiency in densely populated urban environments is obtained. According to that arrangement, each cell is split into six radial sectors and frequencies are assigned to the sectors in such a manner as to provide the ability to reuse each available frequency in every third cell. Although this frequency reuse scheme is highly efficient, it requires at least two complete sets of MULTICARRIER transceiver equipment such as in the form of a broadband base transceiver system (BTS) to be located in each cell. Such a configuration results in dramatically increased hardware installation costs for each cell.
While this equipment is cost effective to deploy when a relatively large number of active mobile stations is expected in each cell, it is not particularly cost effective in most other situations. For example, during an initial system build-out phase, a service provider does not actually need to use large numbers of radio channels. It is therefore typically not possible to justify the cost of deploying complex MULTICARRIER broadband transceiver system equipment based only upon the initial number of subscribers. As a result, the investment in broadband MULTICARRIER radio equipment may not be justified until such time as the number of subscribers increases to a point where the channels are busy most of the time. Furthermore, many areas exist where the need for wireless communication systems is considerable, but where signal traffic can be expected to remain low indefinitely (such as in rural freeway locations or large commercial/industrial parks). Because only a few cells at high expected traffic demand locations (such as in a downtown urban location or a freeway intersection) will justify the initial expense of building out a network of high capacity broadband transceiver systems, the service provider is faced with a dilemma. He can build-out the system with less expensive narrowband equipment initially, to provide some level of coverage, and then upgrade to the more efficient equipment as the number of subscribers rapidly increases in the service area. However, the initial investment in narrowband equipment is then lost. Alternatively, a larger up front investment can be made to deploy the high capacity equipment at the beginning, so that once demand increases, the users of the system can be accommodated without receiving busy signals and the like. But this has the disadvantage of carrying the money cost of a larger up-front investment.
Some have proposed various techniques for expanding the service area of a master cell site. For example, the HPT Cell Site Expander product manufactured by 3 dbm, Inc., of Camarillo, Calif. consists of a base station translator which samples downlink signal traffic and translates it to a selected offset frequency. The offset carrier is transmitted to an expansion cell site via directional antennas. At the expansion cell site, the carrier is translated back to the original cellular channel and transmitted throughout the expansion cell site coverage area such as via an omnidirectional antenna. In the uplink direction, a cellular signal received, by the expansion cell site from a mobile unit is translated and then transmitted back to the base station translator, which in turn translates the signal back to its original carrier frequency.
However, such a device is designed only for use with analog-type cellular systems. A specific problem is encountered when attempting to extend the service area of a base station that uses Time Division Multiple Access (TDMA) signaling. Such a system makes use of a technique in which multiple voice or data channels are provided by dividing the access to each radio carrier frequency into carefully synchronized time slots. In order to properly demodulate a TDMA signal at the base station, a timing advance must be taken into consideration for each radio pulse received from the mobile stations. The timing advance serves to compensate for the differences in signal propagation time since the distance to the base station is different for each mobile station.
A TDMA signal transmitted in the uplink direction must therefore arrive at the Base Transceiver System with proper time alignment. If this is not the case, the signal pulses from the various mobile stations will collide, and it will not be possible for the Base Transceiver System to properly demodulate the signals. As such, it has in most instances been necessary to limit the nominal radius of a TDMA cell so that proper time alignment may be maintained.
An approach to extending the radius of a TDMA cell was disclosed in U.S. Pat. No. 5,544,171, issued to Goedecker and assigned to Alcatel N.V. This technique uses a fixed Base Transceiver System (BTS) that includes both a standard TDMA radio receiver and an additional auxiliary TDMA receiver. The auxiliary TDMA receiver receives and compensates the TDMA radio pulses from mobile stations located outside of the nominal cell radius. In this manner, interference between the TDMA signals received from a mobile station located outside of the nominal cell radius and a mobile station located within the nominal radius is avoided.
Unfortunately, the Goedecker technique is intended for use where both radio transceivers can be located entirely within the base station site. This permits the timing signals for the auxiliary TDMA receiver to be directly connected to the timing signals for the standard TDMA receiver. Thus, it would not be possible to directly apply the Goedecker technique to a remote repeater or translator arrangement, where the auxiliary TDMA receiver would have to be located many miles away from the base station site and such timing signal connection would not be possible.
Furthermore, while the HPT and Goedecker designs can be used to extend the radius of a single cell, they do not appear to suggest how to synchronize TDMA signals received from multiple mobile stations located in multiple cells simultaneously, nor do they suggest any form of random access control channel processing of initial uplink transmissions from mobile stations.
It is an object of this invention to extend the available range in a cellular communication system beyond that which is normally available with Time Division Multiple Access (TDMA) air interfaces.
Another object is to provide for time delay compensation in TDMA systems without using multiple auxiliary receivers.
A further object is to compensate for the delay associated between a translating receiver deployed in a remote outlying cell and a host base station.
Yet another object is to provide for remote receiver time delay compensation in an uplink direction by measuring a delay observed in a downlink direction.
It is still another object of this invention to provide accurate diversity for Random Access Control Channel bursts in the initial uplink transmission.
Briefly, the invention is an architecture for a wireless communication system in which the cells are grouped into clusters. A host cell location is identified within each cluster and a MULTICARRIER broadband Base Transceiver System (BTS) is located at or near the host cell site.
Rather than deploy a complete suite of base station equipment at each remaining cell in the cluster, translating radio transceivers are located in the remote cells. These translating radio transceivers operate in-band, that is, within the frequencies assigned to the service provider.
The in-band translators operate in both an uplink and downlink direction. That is, uplink signals transmitted by a mobile station located in a remote cell are received at the in-band translator, translated to a different carrier frequency, and then transmitted to the host BTS. Likewise, downlink signals transmitted by the host BTS are first received by the in-band translator, translated to a different carrier frequency, and then repeated out to the mobile stations.
The host BTS measures a time delay for each in-band translator channel during a calibration mode. This is accomplished by setting the in-band translator to a xe2x80x9cloopbackxe2x80x9d mode whereby the downlink signal received from the host BTS and transmitted to the mobile station is looped back to both of the uplink receive paths. A timing test signal in the form of, for example, a random access control channel (RACCH) burst is then transmitted by the host BTS such as would normally be sent by a mobile station. The RACCH burst is received by the in-band translator and looped back to the host BTS. The host BTS then demodulates the looped back signal, and measures the elapsed time interval between the transmission and reception of the loopback signal at the host BTS. A resulting time-of-arrival delay estimate as measured in the downlink path is then calculated and used by the host BTS to compensate for time alignments to be made in the time slots in the uplink signal during normal operation.
As a result, the limitation on the backhand range of the cell site normally associated with Time Division Multiple Access protocols is avoided. Indeed, the backhand range of such a system is limited only by the expected attenuation in the radio link.
In accordance with the invention, the in-band translator continually monitors the time slots and multiframe bursts in the signals transmitted by the BTS. A continuous, or watchdog timer routine will execute each time that the time slot (physical channel) is detected. By counting subsequent time slots and multiframe bursts, the in-band translator is able to recognize when a specific time slot is capable of supporting a RACCH burst from the mobile station, and accordingly can provide diversity at the appropriate time slot.
As a result, the in-band translator compensates for timing differences of RACCH bursts between randomly positioned mobile stations and the host BTS.